Sense amplifier

ABSTRACT

Embodiments include a sense amplifier circuit including first and second paths that may be selectively coupled to a memory cell or a reference cell as part of a two-phase read process. The sense amplifier may include a biasing circuit to provide an adaptive bias voltage to a transistor of the first and/or second path to cause the transistor to provide a voltage across the memory cell and/or reference cell that is substantially constant across process corners. Additionally, or alternatively, the sense amplifier may include a DC-coupled regenerative latch circuit to generate a digital output signal based on a voltage difference between nodes of the first and second paths at or near the end of the second phase. Additionally, or alternatively, trimmable offset resistors may adjust a resistance value provided to the sense amplifier by the memory cell and/or reference cells. Other embodiments may be described and claimed.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a divisional of U.S. patent application Ser.No. 14/864,702, filed Sep. 24, 2015, entitled “SENSE AMPLIFIER,” theentire disclosure of which is hereby incorporated by reference in itsentirety.

FIELD

Embodiments of the present invention relate generally to the technicalfield of electronic circuits, and more particularly to sense amplifiers.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure. Unless otherwise indicated herein, the approaches describedin this section are not prior art to the claims in the presentdisclosure and are not admitted to be prior art by inclusion in thissection.

In resistive memory devices, such as magnetic random access memory(MRAM) devices, memory cells have a programmable resistance that is usedto store data. For example, a memory cell may have a first resistance tostore a bit having a first value or a second resistance to store a bithaving a second value. The value of the bit stored by the memory cellmay be read by dropping a voltage across the memory cell and reading theresulting current. For example, a sense amplifier may compare thecurrent through the memory cell with a reference current.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be readily understood by the following detaileddescription in conjunction with the accompanying drawings. To facilitatethis description, like reference numerals designate like structuralelements. Embodiments are illustrated by way of example and not by wayof limitation in the figures of the accompanying drawings.

FIG. 1 schematically illustrates a sense amplifier in accordance withvarious embodiments.

FIG. 2A is a graph showing values of a Vclamp voltage, a bitlinevoltage, and a magnetic tunnel junction (MTJ) voltage for a senseamplifier that uses a pre-determined fixed value for the Vclamp voltage,in accordance with various embodiments.

FIG. 2B is a graph showing values of a Vclamp voltage, a bitlinevoltage, and an MTJ voltage for a sense amplifier that uses an adaptiveVclamp voltage, in accordance with various embodiments.

FIG. 3 illustrates an example system configured to employ theapparatuses and methods described herein, in accordance with variousembodiments.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings that form a part hereof wherein like numeralsdesignate like parts throughout, and in which is shown by way ofillustration embodiments that may be practiced. It is to be understoodthat other embodiments may be utilized and structural or logical changesmay be made without departing from the scope of the present disclosure.Therefore, the following detailed description is not to be taken in alimiting sense, and the scope of embodiments is defined by the appendedclaims and their equivalents.

Various operations may be described as multiple discrete actions oroperations in turn, in a manner that is most helpful in understandingthe claimed subject matter. However, the order of description should notbe construed as to imply that these operations are necessarily orderdependent. In particular, these operations may not be performed in theorder of presentation. Operations described may be performed in adifferent order than the described embodiment. Various additionaloperations may be performed and/or described operations may be omittedin additional embodiments.

For the purposes of the present disclosure, the phrases “A and/or B” and“A or B” mean (A), (B), or (A and B). For the purposes of the presentdisclosure, the phrase “A, B, and/or C” means (A), (B), (C), (A and B),(A and C), (B and C), or (A, B, and C).

The description may use the phrases “in an embodiment,” or “inembodiments,” which may each refer to one or more of the same ordifferent embodiments. Furthermore, the terms “comprising,” “including,”“having,” and the like, as used with respect to embodiments of thepresent disclosure, are synonymous.

As used herein, the term “circuitry” may refer to, be part of, orinclude an Application Specific Integrated Circuit (ASIC), an electroniccircuit, a processor (shared, dedicated, or group), and/or memory(shared, dedicated, or group) that execute one or more software orfirmware programs, a combinational logic circuit, and/or other suitablehardware components that provide the described functionality. As usedherein, “computer-implemented method” may refer to any method executedby one or more processors, a computer system having one or moreprocessors, a mobile device such as a smartphone (which may include oneor more processors), a tablet, a laptop computer, a set-top box, agaming console, and so forth.

Embodiments include a sense amplifier circuit to read a value of a bitstored by a memory cell. In some embodiments, the memory cell may be aresistive memory cell, such as a spin-transfer torque (STT) magneticrandom access memory (MRAM) cell. The sense amplifier circuit mayinclude a first signal path and a second signal path having respectivefirst or second intermediate nodes disposed between respectivetransistors. During a first phase of the read operation, switchcircuitry may selectively couple a first reference cell to the firstsignal path and selectively couple the memory cell to the second signalpath. During a second phase of the read operation, the switch circuitrymay selectively couple the memory cell to the first signal path and mayselectively couple a second reference cell to the second signal path.The sense amplifier may generate a differential voltage between thefirst and second intermediate nodes having a value that is based on thevalue of the bit stored by the memory cell.

In various embodiments, the sense amplifier may include a biasingcircuit to provide an adaptive clamp voltage to a transistor of thefirst and/or second path to cause the transistor to provide a voltageacross the memory cell and/or reference cells that is substantiallyconstant across process and supply voltage (Vcc) corners. Additionally,or alternatively, the sense amplifier may include a DC-coupledregenerative latch circuit to generate a digital output signal based ona voltage difference between the first and second intermediate nodes ator near the end of the second phase. The latch circuit may be DC-coupledbetween the intermediate nodes on the first and second paths.Additionally, or alternatively, the sense amplifier may includetrimmable offset resistors coupled to the input terminals to adjust aresistance value provided to the sense amplifier by the memory celland/or reference cells. The resistances of the trimmable offsetresistors may be adjusted to balance read margins of the senseamplifier.

FIG. 1 illustrates a sense amplifier circuit 100 (hereinafter “circuit100”), in accordance with various embodiments. The sense amplifiercircuit 100 may be coupled to one or more memory cells (e.g., memorycell 102) of a memory to read data that is stored by the memory cells.The sense amplifier 100 may be included in a read circuit. In someembodiments, the memory cells may be resistive memory cells, such asspin-transfer torque (STT) magnetic random access memory (MRAM) cells,that store data based on a resistive state of the memory cells. Forexample, individual STT-MRAM memory cells may include a magnetic tunneljunction (MTJ) having a dielectric layer (e.g., oxide layer) disposedbetween a fixed magnet layer and a free magnet layer. The MTJ may storedata according to the orientation of the free magnet. For example, thefree magnet may be programmed to a parallel state with the fixed magnetlayer to store a bit with a first logic value (e.g., logic 1) or ananti-parallel state with the fixed magnet layer to store a second logicvalue (e.g., a logic 0). The MTJ may have a higher resistance in theanti-parallel state than in the parallel state. The MTJ may beprogrammed to the parallel state or the anti-parallel state based on adirection of current that is passed through the MTJ during the writeprocess.

In various embodiments, a memory cell 102 may be coupled to input dataterminals 104 and 106 of the circuit 100. The circuit 100 may furtherinclude a first reference terminal 108 (REF0) and a second referenceterminal 110 (REF1). The first reference terminal 108 may be coupled toa first reference cell 112, and the second reference terminal 110 may becoupled to a second reference cell 114. In some embodiments, the firstreference cell 112 and second reference cell 114 may have differentresistances. For example, the first reference cell 112 may have a lowerresistance than the second reference cell 114. Alternatively, the firstreference cell 112 may have a higher resistance than the secondreference cell 114.

In some embodiments, the first reference cell 112 may have a resistancethat corresponds to the low resistance state of the memory cell 102(e.g., the resistance in the parallel state), and the second referencecell 114 may have a resistance that corresponds to the high resistancestate of the memory cell 102 (e.g., the resistance in the anti-parallelstate). Alternatively, the first reference cell 112 may have aresistance that corresponds to the high resistance state of the memorycell 102, and the second reference cell 114 may have a resistance thatcorresponds to the low resistance state of the memory cell 102. Asdiscussed further below, the first reference cell 112 and secondreference cell 114 may combine to provide a mid-point referenceresistance that is between the low resistance state and the highresistance state of the memory cell 102.

In various embodiments, the circuit 100 may have a first path thatincludes transistors MP201, MP202, and MN200 that are coupled to oneanother in series along the first path, and a second path that includesMP211, MP212, and MN210 that are coupled to one another in series alongthe second path. As used herein, transistors designated as MP may bep-type transistors (e.g., p-type metal-oxide-semiconductor (PMOS))transistors, and transistors designated as MN may be n-type transistors(e.g., n-type metal-oxide-semiconductor (NMOS) transistors). However, itwill be apparent that different transistor types may be used in someembodiments without departing from the scope of the present disclosure.

In various embodiments, a first intermediate node INT0 may be disposedbetween the transistor MN200 and transistors MP202 and MP201 on thefirst path. A second intermediate node INT1 may be disposed between thetransistor MN210 and transistors MP212 and MP211 on the second path.

In various embodiments, a switch transistor MN101 may be coupled betweenthe first input data terminal 104 and transistor MN200 of the first pathto selectively provide a conductive path between the memory cell 102 andthe first path, and a switch transistor MN100 may be coupled between thesecond input data terminal 106 and transistor MN210 of the second pathto selectively provide a conductive path between the memory cell 102 andthe second path. The switch transistor MN100 may be controlled by afirst control signal (Φ1), and the switch transistor MN101 may becontrolled by a second control signal (Φ2).

A switch transistor MN110 may be coupled between the first referenceterminal 108 and transistor MN200 of the first path to selectivelyprovide a conductive path between the reference cell 112 and the firstpath, and a switch transistor MN111 may be coupled between the secondreference terminal 110 and transistor MN210 of the second path toselectively provide a conductive path between the reference cell 114 andthe second path. The switch transistor MN110 may be controlled by thefirst control signal Φ1 and the switch transistor MN111 may becontrolled by the second control signal Φ2.

In various embodiments, the circuit 100 may further include a switchTG200 coupled between the gate terminal of the transistor MP201 and thedrain terminal of transistor MP202, a switch TG201 coupled between thegate terminal and the drain terminal of the transistor MP202, a switchTG210 coupled between the gate terminal of the transistor MP211 and thedrain terminal of transistor MP212, and/or a switch TG211 coupledbetween the gate terminal and the drain terminal of transistor MP212.The switches TG200, TG201, TG210, and TG211 may be controlled by thefirst control signal Φ1. The circuit 100 may further include a capacitorC200 coupled to the gate terminal of the transistor MP201, a capacitorC201 coupled to the gate terminal of the transistor MP202, a capacitorC210 coupled to the gate terminal of the transistor MP211, and acapacitor C211 coupled to the gate terminal of the transistor MP212.

In various embodiments, a bias circuit 116 may be coupled with the gateterminals of the transistor MP201 and transistor MN200. The bias circuit116 may generate a bias voltage (also referred to as clamp voltage),Vclamp0, and pass the bias voltage to the gate terminal of thetransistor MN200. The bias voltage may have a value that varies withprocess conditions of one or more transistors of the circuit 100. Forexample, in some embodiments, the bias circuit 116 may be a currentmirror to copy the current through the transistor MP201 and pass thecopied current through a transistor (e.g., a diode-connected transistor)to provide a voltage Vclamp0 at the gate terminal of the transistorMN200. The voltage Vclamp0 may be a constant voltage with a value thatis based on the threshold voltage of the diode-connected transistor.Accordingly, the value of the voltage Vclamp0 may adjust to variation inthreshold voltages across process corners.

For example, in some embodiments, the bias circuit 116 may include atransistor MP50 having a gate terminal that is coupled to the gateterminal of the transistor MP201, and a diode-connected transistor MN50having a drain terminal and gate terminal coupled with the gate terminalof the transistor MN200. The transistor MP50 may copy the current oftransistor MP201 and pass the copied current through the transistorMN50. The voltage Vclamp0 at the gate terminal of the transistor MN200may be based on the threshold voltage of the transistor MN50.Accordingly, the voltage Vclamp0 may adapt to variation in thetransistors of circuit 100 across process corners to provide a voltageat the source terminal of transistor MN200 that is more stable acrossprocess corners than using a pre-determined fixed value for the voltageVclamp0.

In some embodiments, the bias circuit 116 may further include a resistorR50 coupled between the source terminal of the transistor MN50 andground. Resistor R50 may be used to trim the value of Vclamp.Additionally, or alternatively, the resistors R50 of different senseamplifier circuits 100 that are coupled to different bit lines may havedifferent resistance values to mimic the voltage drop that may occuracross the bit lines.

In various embodiments, the circuit 100 may further include another biascircuit 118 coupled to the gate terminals of the transistors MP211 andMN210. For example, the bias circuit 118 may include transistor MP51,diode-connected transistor MN51, and/or resistor R51 coupled to oneanother as shown. The bias circuit 118 may copy the current of thetransistor MP211 and pass the current through the transistor MN51 togenerate a voltage Vclamp1 at the gate terminal of the transistor MN210.The value of the voltage Vclamp1 may be based on the threshold voltageof the transistor MN51.

In some embodiments, the gate terminal of the transistor MN210 may becoupled with the gate terminal of the transistor MN200. Accordingly, thevoltages Vclamp0 and Vclamp1 may have a same value (e.g., thatcorresponds to an average of the voltage provided by transistor MN50 andthe voltage provided by MN51). With the gate terminals of transistorsMN200 and MN210 coupled together, the Vclamp0 and Vclamp1 voltages maybe more stable.

In some embodiments, the gate terminals of the transistors MN200 andMN210 may additionally or alternatively be coupled to the gate terminalsof the corresponding transistors in one or more other sense amplifiercircuits of the read circuit (e.g., that are associated with otherbitlines). However, coupling the gate terminals of the transistors MN200and MN210 across multiple sense amplifier circuits brings the risk thata bit error in one sense amplifier circuit may cause bit errors for oneor more of the other sense amplifier circuits.

With the adaptive Vclamp0 and Vclamp1 voltages, the resulting voltage atthe bitline (e.g., at input data terminals 104 and/or 106) and/or theresulting voltage that is dropped across the memory cell 102 may haveless variation across process corners than if a pre-determined fixedvalue is used for the Vclamp0 and Vclamp1 voltages. For example, FIG. 2Aillustrates a graph 200 showing values 202 for the Vclamp voltage,values 204 for the bitline voltage, and values 206 for the MTJ voltagefor various process conditions for a sense amplifier circuit that uses aconstant Vclamp voltage (e.g., 0.402 Volts) for all the processconditions. The values 204 for the bitline voltage vary from 0.156 Voltsto 0.055 Volts, and the values 206 for the MTJ voltage vary from 0.117Volts to 0.039 Volts.

FIG. 2B illustrates a graph 210 showing values 212 for the Vclampvoltage, values 214 for the bitline voltage, and values 216 for the MTJvoltage across various process conditions for a sense amplifier circuitthat uses an adaptive Vclamp voltage as described herein (e.g., asprovided by the current mirrors 116 and/or 118). The values 212 of theVclamp voltage vary from 0.348 Volts to 0.475 Volts. The resultingvalues 214 of the bitline voltage vary from 0.118 Volts to 0.099 Volts,and the resulting values 216 of the MTJ voltage vary from 0.086 Volts to0.068 Volts. Accordingly, as illustrated by FIGS. 2A and 2B, the valuesof the bitline voltage and MTJ voltage may be more consistent acrossprocess corners when using an adaptive Vclamp voltage than when using afixed value for the Vclamp voltage.

In various embodiments, the circuit 100 may perform a multi-phase readprocess to determine the value of the bit stored by the memory cell 102.The multi-phase read process may cancel out signal offset that may becaused by variation in the transistors MP201, MP202, and MN200 on thefirst path and transistors MP211, MP212, and MN210 on the second path.The multi-phase read operation may include first and second phases. Insome embodiments, the first phase may be referred to as a samplingphase, and the second phase may be referred to as an amplificationphase.

During the first phase of the read process, the first control signal Φ1may have a first logic value (e.g., logic high) to turn on switchtransistors MN100 and MN110, and switches TG200, TG201, TG210, andTG211. The second control signal Φ2 may have a second logic value (e.g.,logic low) such that transistors MN101 and MN111 are off. Accordingly,the reference cell 112 may be conductively coupled to the first path,and the memory cell 102 may be conductively coupled to the second path.Additionally, the capacitors C200 and C201 may be conductively coupledto the intermediate node INTO by respective switches TG200 and TG201,and capacitors C210 and C211 may be conductively coupled to theintermediate node INT1 by respective switches TG210 and TG211.Therefore, the capacitors C200 and C201 may sample the current on thefirst path and store a corresponding charge. The current sampled by thecapacitors C200 and C201 may correspond to the current through thereference cell 112. The capacitors C210 and C211 may sample the currenton the second path and store a corresponding charge. The current on thesecond path may correspond to the current through the memory cell 102(e.g., having a value based on the resistive state of the memory cell102). A difference between the first current contribution of the firstpath and the second current contribution of the second path may bereferred to as an offset current.

During the second phase of the read process, the first control signal Φ1may have the second logic value such that transistors MN100 and MN110,and switches TG200, TG201, TG210, and TG211 are off. The second controlsignal Φ2 may have the first logic value such that switch transistorsMN101 and MN111 are on (e.g., conducting). Accordingly, the memory cell102 may be conductively coupled with the first path via the switchtransistor MN101, and reference cell 114 may be conductively coupledwith the second path via the switch transistor MN111.

The voltage provided by transistor MN200 (e.g., based on the biasvoltage Vclamp0) may cause a current I_(CELL) to pass through the memorycell 102. The charge stored by capacitors C200 and C201 may causetransistors MP201 and MP202 to provide a current I_(REF0) thatcorresponds to the current through the first reference cell 112 in thefirst phase. Accordingly, the voltage at the intermediate node INT0, V1may be V1=R_(OUT)*(I_(REF0)−I_(CELL)), where R_(OUT) is the resistanceseen at intermediate node INT0 (which may be the same as the resistanceseen at intermediate node INT0.

The voltage provided by transistor MN210 may cause a current I_(REF1) topass through the reference cell 114. The charge stored by capacitorsC210 and C211 may cause transistors MP211 and MP212 to provide a currentthat corresponds to the I_(CELL) current through the memory cell 102 (assampled during the first phase). Accordingly, the voltage at theintermediate node INT1, V2, may be V2=R_(OUT)*(I_(CELL)−I_(REF1)).

Therefore, the voltage difference, ΔV, between the intermediate nodesINT0 and INT1 may beΔV=V2−V1=2*(I_(CELL)−(I_(REF0)+I_(REF1))/2)*R_(OUT). The contributionfrom the offset voltage caused by differences in the first and secondpaths may be cancelled out. The voltage difference ΔV may be positivefor the first resistive state of the memory cell and may be negative forthe second resistive state of the memory cell.

In various embodiments, a latch circuit 120 may be direct current (DC)coupled between the intermediate nodes INT0 and INT1 (e.g., with a DCconductive path between the intermediate nodes INT0 and INT1 and thelatch circuit 120). The latch circuit 120 may generate a digital outputsignal based on the voltage difference between the intermediate nodesINT0 and INT1 at the end of the second phase. For example, the latchcircuit 120 may force one of the intermediate nodes INT0 or INT1 to avalue corresponding to a logic 0 (e.g., a voltage of about 0 Volts(ground)) and may force the other of the intermediate nodes INT0 or INT1to a value corresponding to a logic 1 (e.g., a voltage of about Vcc)based on the voltage difference ΔV. If the voltage at INT0 is higherthan the voltage at INT1, the latch circuit 120 may force INT0 to alogic 1 and INT1 to a logic 0. If the voltage at INTO is less than thevoltage at INT1, the latch circuit 120 may force INT0 to a logic 0 andINT1 to a logic 1.

In some embodiments, the latch circuit 120 may include pull-uptransistors MP30 and MP31, and pull-down transistors MN30 and MN31. Thepull-up transistor MP30 may be coupled between the first intermediatenode INT0 and a supply terminal 122, and pull-down transistor MN30 maybe coupled between the first intermediate node INT0 and a groundterminal 124. The pull-up transistor MP31 may be coupled between thesecond intermediate node INT1 and the supply terminal 122, and pull-downtransistor MN31 may be coupled between the second intermediate node INT1and the ground terminal 124. The gate terminals of the transistors MP30and MN30 may be coupled to the second intermediate node INT1 and thegate terminals of the transistors MP31 and MN31 may be coupled to thefirst intermediate node INT0.

In some embodiments, the latch circuit 120 may further include an enabletransistor MP3 coupled between the pull-up transistors and the supplyterminal 122 and an enable transistor MN3 coupled between the pull-downtransistors MN30 and MN31 and the ground terminal 124. The enabletransistor MN3 may receive an enable signal SAEN, and the enabletransistor MP3 may receive an enable bar signal SAENB (which may be theinverse of the enable signal SAEN) to selectively activate the latchcircuit 120. The enable signals SAEN and/or SAENB may activate the latchcircuit 120 at or near the end of the second phase.

When the latch circuit 120 is activated, the contention from thecross-coupling of the transistors of the latch circuit 120 may force oneof the intermediate nodes INT0 or INT1 to a value corresponding to alogic 0 (e.g., a voltage of about 0 Volts (ground)) and may force theother of the intermediate nodes INT0 or INT1 to a value corresponding toa logic 1 (e.g., a voltage of about Vcc) based on the voltage differenceΔV. For example, if the voltage at INT0 is higher than the voltage atINT1, the pull-up transistor MP30 may turn on (and the pull-downtransistor MN30 may be off) to couple the node INT0 to the supplyterminal 122 and bring the voltage at node INT0 to about Vcc. Thepull-down transistor MN31 may turn on (and the pull-up transistor MP31may be off) to couple the node INT1 to the ground terminal 124 and bringthe voltage at node INT1 to about 0 Volts. Alternatively, if the voltageat INT0 is less than the voltage at INT1, the pull-down transistor MN30may turn on (and the pull-up transistor MP30 may be off) to couple thenode INT0 to the ground terminal 124 and bring the voltage at node INT0to about 0 Volts. The pull-up transistor MP31 may turn on (and thepull-down transistor MN31 may be off) to couple the node INT1 to thesupply terminal 122 and bring the voltage at the node INT1 to about Vcc.

The voltages at the intermediate nodes INT0 and INT1 may be passed asthe output signal of the circuit 100 (e.g., at output terminals OUT0 andOUT1). In some embodiments, the circuit 100 may further include NANDgates NA30 and NA31. The input terminals of the NAND gate NA30 may becoupled with the first intermediate node INT0 and the enable signalSAEN, respectively. The input terminals of the NAND gate NA31 may becoupled with the second intermediate node INT1 and the enable signalSAEN, respectively. Accordingly, when the enable signal SAEN is on(e.g., high), the NAND gates NA30 and NA31 may pass an inverted versionof the logic value at the respective intermediate node INT0 or INT1 tothe respective output terminals OUT0 and OUT1.

The latch circuit 120 may be included in a second stage of the circuit100 that generates a digital output signal based on the voltagedifference ΔV generated at the intermediate nodes INT0 and INT1 by thefirst stage.

In various embodiments, the circuit 100 may further include a transistorMP2 (also referred to as a cutoff transistor) coupled between the sourceterminals of the transistors MP201 and MP211. A transistor MP200 mayhave a drain terminal coupled to the source terminal of the transistorMP211 of the second path, a gate terminal coupled to the firstintermediate node INT0, and a source terminal coupled to receive thesupply voltage (e.g., Vcc). A transistor MP210 may have a drain terminalcoupled to the source terminal of the transistor MP201 of the firstpath, a gate terminal coupled to the second intermediate node INT1, anda source terminal coupled to receive the supply voltage. The transistorsMP200 and MP201 may provide common mode feedback to regulate the voltagelevels at the intermediate nodes INT0 and INT1.

In various embodiments, the transistor MP2 may receive a latch signal toturn off the transistor MP2 at the end of the second phase of the readprocess. In some embodiments, a NAND gate NA4 may have input terminalscoupled to the respective output terminals OUT0 and OUT1 and an outputterminal coupled to the gate terminal of the transistor MP2 to providethe latch signal. When the second stage is activated, the outputterminals OUT0 and OUT1 may be driven to 0 and 1 or 1 and 0,respectively, causing the NAND gate NA4 to turn off transistor MP2.Accordingly, the DC path between the first stage and second stage may becut off.

Furthermore, the resistance, Rdson, between the drain and sourceterminals of the transistor MP2 when the transistor MP2 is on mayintroduce a controlled positive feedback to enhance the gain of thefirst stage and thereby increase the voltage difference between theintermediate nodes INT0 and INT1 generated by the first stage.Accordingly, the input offset requirements of the second stage may berelaxed.

In some embodiments, the circuit 100 may further include trimmableresistors coupled between the memory cell 102, reference cell 112,and/or reference cell 114 and the respective switch transistors MN101,MN100, MN110, and MN111. For example, circuit 100 may include a resistorR100 coupled between the transistor MN100 and the input data terminal106, a resistor R101 coupled between transistor MN101 and input dataterminal 104, a resistor R110 coupled between the transistor MN110 andthe reference terminal 108, and/or a resistor R111 coupled between thetransistor MN111 and the reference terminal 110. The values of theresistors R100, R101, R110, and/or R111 may be adjustable. For example,in some embodiments, the resistors R100 and R101 may have a firstresistance value OFFSET0, and the resistors R110 and R111 may have asecond resistance value OFFSET1. The resistance values of the resistorsR100, R101, R110, and/or R111 may add to the resistance value of thecorresponding memory cell 102, reference cell 112, or reference cell 114to affect the value of the current through the memory cell 102,reference cell 112, or reference cell 114 during the read operation.

In various embodiments, the resistance values of the resistors R100,R101, R110, and/or R111 (e.g., OFFSET0 and OFFSET1) may be set tobalance the read margins for reading a logic 1 bit and reading a logic 0bit from the memory cell 102. For example, the resistance values of theresistors R100, R101, R110, and/or R111 may be determined such that anumber and/or frequency of read errors for reading bits from the memorycell 102 is about the same for reading a logic 1 bit as for reading alogic 0 bit. The resistance values of the resistors R100, R101, R110,and/or R111 may be determined by a control circuit, such as memorycontroller 305 of system 300.

Although the circuit 100 is described herein with memory cell 102 beinga resistive memory cell, the circuit 100 may be used to read data fromother types of memory cells and/or sensors that may act as a currentsource at data input terminals 104 and 106. In embodiments in which thememory cell 102 acts as a current source, the offset resistors R100,R101, R110, and/or R111 may be replaced with current sources that arecoupled in parallel with the respective memory cell or reference cell.

FIG. 3 illustrates an example computing device 300 that may employ theapparatuses and/or methods described herein (e.g., circuit 100), inaccordance with various embodiments. As shown, computing device 300 mayinclude a number of components, such as one or more processor(s) 304(one shown) and at least one communication chip 306. In variousembodiments, the one or more processor(s) 304 each may include one ormore processor cores. In various embodiments, the at least onecommunication chip 306 may be physically and electrically coupled to theone or more processor(s) 304. In further implementations, thecommunication chip 306 may be part of the one or more processor(s) 304.In various embodiments, computing device 300 may include printed circuitboard (PCB) 302. For these embodiments, the one or more processor(s) 304and communication chip 306 may be disposed thereon. In alternateembodiments, the various components may be coupled without theemployment of PCB 302.

Depending on its applications, computing device 300 may include othercomponents that may or may not be physically and electrically coupled tothe PCB 302. These other components include, but are not limited to,memory controller 305, volatile memory (e.g., dynamic random accessmemory (DRAM) 308), non-volatile memory such as read only memory (ROM)310, flash memory 312, storage device 311 (e.g., a hard-disk drive(HDD)), an I/O controller 314, a digital signal processor (not shown), acrypto processor (not shown), a graphics processor 316, one or moreantenna 318, a display (not shown), a touch screen display 320, a touchscreen controller 322, a battery 324, an audio codec (not shown), avideo codec (not shown), a global positioning system (GPS) device 328, acompass 330, an accelerometer (not shown), a gyroscope (not shown), aspeaker 332, a camera 334, and a mass storage device (such as hard diskdrive, a solid state drive, compact disk (CD), digital versatile disk(DVD)) (not shown), and so forth. In various embodiments, the processor304 may be integrated on the same die with other components to form aSystem on Chip (SoC).

In some embodiments, the one or more processor(s) 304, flash memory 312,and/or storage device 311 may include associated firmware (not shown)storing programming instructions configured to enable computing device300, in response to execution of the programming instructions by one ormore processor(s) 304, to practice all or selected aspects of themethods described herein. In various embodiments, these aspects mayadditionally or alternatively be implemented using hardware separatefrom the one or more processor(s) 304, flash memory 312, or storagedevice 311.

In various embodiments, one or more components of the computing device300 may include the circuit 100 described herein. For example, thecircuit 100 may be included in the memory controller 305, processor 304,I/O controller 314, and/or another component of the computing device300. In some embodiments, one or more of the DRAM 308, flash memory 312,ROM 310, and/or storage device 311 may include memory cells that may beread by the circuit 100.

The communication chips 306 may enable wired and/or wirelesscommunications for the transfer of data to and from the computing device300. The term “wireless” and its derivatives may be used to describecircuits, devices, systems, methods, techniques, communicationschannels, etc., that may communicate data through the use of modulatedelectromagnetic radiation through a non-solid medium. The term does notimply that the associated devices do not contain any wires, although insome embodiments they might not. The communication chip 306 mayimplement any of a number of wireless standards or protocols, includingbut not limited to IEEE 702.20, Long Term Evolution (LTE), LTE Advanced(LTE-A), General Packet Radio Service (GPRS), Evolution Data Optimized(Ev-DO), Evolved High Speed Packet Access (HSPA+), Evolved High SpeedDownlink Packet Access (HSDPA+), Evolved High Speed Uplink Packet Access(HSUPA+), Global System for Mobile Communications (GSM), Enhanced Datarates for GSM Evolution (EDGE), Code Division Multiple Access (CDMA),Time Division Multiple Access (TDMA), Digital Enhanced CordlessTelecommunications (DECT), Worldwide Interoperability for MicrowaveAccess (WiMAX), Bluetooth, derivatives thereof, as well as any otherwireless protocols that are designated as 3G, 4G, 5G, and beyond. Thecomputing device 300 may include a plurality of communication chips 306.For instance, a first communication chip 306 may be dedicated to shorterrange wireless communications such as Wi-Fi and Bluetooth, and a secondcommunication chip 306 may be dedicated to longer range wirelesscommunications such as GPS, EDGE, GPRS, CDMA, WiMAX, LTE, Ev-DO, andothers.

In various implementations, the computing device 300 may be a laptop, anetbook, a notebook, an ultrabook, a smartphone, a computing tablet, apersonal digital assistant (PDA), an ultra-mobile PC, a mobile phone, adesktop computer, a server, a printer, a scanner, a monitor, a set-topbox, an entertainment control unit (e.g., a gaming console or automotiveentertainment unit), a digital camera, an appliance, a portable musicplayer, or a digital video recorder. In further implementations, thecomputing device 300 may be any other electronic device that processesdata.

Some non-limiting Examples of various embodiments are presented below.

Example 1 is a sense amplifier circuit comprising: a first pathincluding a first transistor, a second transistor, and an intermediatenode coupled between the first and second transistors; switch circuitryto selectively couple a reference cell to the first path during a firstphase of a read operation and to selectively couple a memory cell to thefirst path during a second phase of the read operation to generate avoltage at the intermediate node that is indicative of a value of a bitstored by the memory cell; and a bias circuit coupled to the first andsecond transistors, the bias circuit to generate a first current basedon a current through the first transistor and pass the first currentthrough a third transistor to provide a bias voltage to a gate terminalof the second transistor.

Example 2 is the circuit of Example 1, wherein the third transistor is adiode-connected transistor.

Example 3 is the circuit of Example 2, wherein the bias circuit includesa fourth transistor with a gate terminal coupled to a gate terminal ofthe first transistor to copy the current of the first transistor.

Example 4 is the circuit of Example 1, wherein the intermediate node isa first intermediate node, wherein the bias circuit is a first biascircuit, wherein the bias voltage is a first bias voltage, and whereinthe circuit further comprises: a second path including a fourthtransistor, a fifth transistor, and a second intermediate node coupledbetween the fourth and fifth transistors; and a second bias circuitcoupled to the fourth and fifth transistors, the second bias circuit togenerate a second current based on a current through the fourthtransistor and pass the second current through a sixth transistor toprovide a second bias voltage to a gate terminal of the fifthtransistor; wherein the switch circuitry is to selectively couple thememory cell to the second path during the first phase and is toselectively couple a second reference cell to the second path during thesecond phase.

Example 5 is the circuit of Example 4, wherein the gate terminals of thesecond and fifth transistors are coupled to one another such that thefirst bias voltage is the same as the second bias voltage.

Example 6 is the circuit of Example 4, further comprising a latchcircuit that is direct current (DC) coupled between the first and secondintermediate nodes, wherein the latch circuit is to selectively activateat or near the end of the second phase to generate a digital outputsignal based on a voltage difference between the first and secondintermediate nodes.

Example 7 is the circuit of Example 6, wherein the latch circuitincludes a first enable transistor coupled between pull-up transistorsof the latch circuit and a supply terminal and a second enabletransistor coupled between pull-down transistors of the latch circuitand a ground terminal, wherein the first and second enable transistorsare to turn on to activate the latch circuit.

Example 8 is the circuit of Example 6, further comprising a cutofftransistor coupled between the first and second paths to turn offresponsive to activation of the latch circuit.

Example 9 is the circuit of Example 8, wherein the cutoff transistor isto provide amplification gain to the first and second intermediate nodesduring the second phase.

Example 10 is the circuit of any one of Examples 1 to 9, furthercomprising a first trimmable resistor coupled between the switchcircuitry and the memory cell and a second trimmable resistor coupledbetween the switch circuitry and the reference cell.

Example 11 is the circuit of any one of Examples 1 to 9, wherein thememory cell is a resistive memory cell.

Example 12 is a memory circuit comprising: a memory cell; and a senseamplifier circuit coupled to the memory cell, wherein the senseamplifier circuit includes: a first path including a first intermediatenode; a second path including a second intermediate node, wherein thesense amplifier circuit is to generate a voltage difference between thefirst and second intermediate nodes that corresponds to a value of a bitstored by the memory cell; and a latch circuit direct current (DC)coupled between the first and second intermediate nodes, wherein thelatch circuit, when activated by an enable signal, is to generate adigital output signal based on the voltage difference between the firstand second intermediate nodes.

Example 13 is the circuit of Example 12, wherein the latch circuitincludes a first enable transistor coupled between pull-up transistorsof the latch circuit and a supply terminal and a second enabletransistor coupled between pull-down transistors of the latch circuitand a ground terminal, wherein the first and second enable transistorsare to turn on responsive to the enable signal to activate the latchcircuit after generation of the voltage difference.

Example 14 is the circuit of Example 13, wherein the pull-up transistorsinclude: a first pull-up transistor having a drain terminal coupled tothe first intermediate node and a gate terminal coupled to the secondintermediate node; and a second pull-up transistor having a drainterminal coupled to the second intermediate node and a gate terminalcoupled to the first intermediate node. Additionally, the pull-downtransistors include: a first pull-down transistor having a drainterminal coupled to the first intermediate node and a gate terminalcoupled to the second intermediate node; and a second pull-downtransistor having a drain terminal coupled to the second intermediatenode and a gate terminal coupled to the first intermediate node.

Example 15 is the circuit of any one of Examples 12 to 14, wherein thesense amplifier further includes a cutoff transistor coupled between thefirst and second paths to turn off responsive to activation of the latchcircuit.

Example 16 is the circuit of Example 15, wherein the sense amplifierfurther includes: a first feedback transistor with a drain terminalcoupled to a drain terminal of the cutoff transistor and a gate terminalcoupled to the first intermediate node; and a second feedback transistorwith a drain terminal coupled to a source terminal of the cutofftransistor and a gate terminal coupled to the second intermediate node.

Example 17 is the circuit of Example 15, wherein the sense amplifierfurther includes: first output logic coupled to the first intermediatenode to selectively pass a first digital value based on the voltage atthe first intermediate node responsive to the enable signal; secondoutput logic coupled to the second intermediate node to selectively passa second digital value based on the voltage at the second intermediatenode responsive to the enable signal, wherein the second digital valueis the inverse of the first digital value; and cutoff logic to receivethe first and second digital values as inputs and having an outputterminal coupled to a gate terminal cutoff transistor to turn off thecutoff transistor responsive to receipt of the first and second digitalvalues.

Example 18 is a memory system comprising: a resistive memory including aresistive memory cell; a reference cell having a first resistance; asense amplifier coupled to the resistive memory cell and the referencecell, the sense amplifier to read data from the resistive memory cellbased on the reference cell; a first trimmable resistor coupled betweenthe sense amplifier and the resistive memory cell; a second trimmableresistor coupled between the sense amplifier and the reference cell; anda control circuit to adjust resistance values of the first trimmableresistor and the second trimmable resistor to equalize read marginsbetween a logic 0 read and a logic 1 read of the memory cell.

Example 19 is the system of Example 18, wherein the reference cell is afirst reference cell, wherein the system further comprises: a secondreference cell with a second resistance that is different from the firstresistance; a third trimmable resistor coupled between the senseamplifier and the second reference cell; wherein the sense amplifier isto read the data from the resistive memory cell based on the first andsecond reference cells; and wherein the control circuit is to adjust aresistance value of the third trimmable resistor to equalize the readmargins.

Example 20 is the system of Example 19, wherein the control circuit isto control the second and third trimmable resistors to have the sameresistance value.

Example 21 is the system of Example 19, wherein the sense amplifierincludes: a first path including a first intermediate node; a secondpath including a second intermediate node; and switch circuitry. Theswitch circuitry is to: during a first phase of the read, couple thefirst reference cell to the first path and couple the resistive memorycell to the second path; and during a second phase of the read, couplethe resistive memory cell to the first path and couple the secondreference cell to the second path to generate a voltage differencebetween the first and second intermediate nodes that corresponds to avalue of a bit stored by the resistive memory cell.

Example 22 is a memory apparatus comprising: means to, during a firstphase of a read operation, couple a reference cell to a first path andcouple a memory cell to a second path; means to, during a second phaseof the read operation, couple the memory cell with the first path togenerate a voltage difference between a first intermediate node on thefirst path and a second intermediate node on the second path thatcorresponds to a value of a bit stored by the memory cell; means togenerate a bias voltage to control a first transistor of the first pathto provide a voltage across the memory cell or reference cell and inducea current in the first path, the bias voltage having a value based on athreshold voltage of a second transistor.

Example 23 is the apparatus of Example 22, wherein the means to generatethe bias voltage is to copy a current on the first path and pass thecopied current through the second transistor to generate the biasvoltage.

Example 24 is the apparatus of Example 22, further comprising means toadjust an offset resistance associated with the memory cell or thereference cell to balance read margins of the read operation.

Example 25 is the apparatus of any one of Examples 22 to 24, furthercomprising means to generate a digital output signal based on thegenerated voltage difference between the first and second intermediatenodes, wherein the means to generate the digital output signal aredirect current (DC) coupled to the first and second intermediate nodes.

Although certain embodiments have been illustrated and described hereinfor purposes of description, this application is intended to cover anyadaptations or variations of the embodiments discussed herein.Therefore, it is manifestly intended that embodiments described hereinbe limited only by the claims.

Where the disclosure recites “a” or “a first” element or the equivalentthereof, such disclosure includes one or more such elements, neitherrequiring nor excluding two or more such elements. Further, ordinalindicators (e.g., first, second, or third) for identified elements areused to distinguish between the elements, and do not indicate or imply arequired or limited number of such elements, nor do they indicate aparticular position or order of such elements unless otherwisespecifically stated.

What is claimed is:
 1. A circuit comprising: a memory cell; and a senseamplifier circuit coupled to the memory cell, wherein the senseamplifier circuit includes: a first path including a first intermediatenode; a second path including a second intermediate node, wherein thesense amplifier circuit is to generate a voltage difference between thefirst and second intermediate nodes that corresponds to a value of a bitstored by the memory cell; and a latch circuit direct current (DC)coupled between the first and second intermediate nodes, wherein thelatch circuit, when activated by an enable signal, is to generate adigital output signal based on the voltage difference between the firstand second intermediate nodes.
 2. The circuit of claim 1, wherein thelatch circuit includes a first enable transistor coupled between pull-uptransistors of the latch circuit and a supply terminal and a secondenable transistor coupled between pull-down transistors of the latchcircuit and a ground terminal, wherein the first and second enabletransistors are to turn on responsive to the enable signal to activatethe latch circuit after generation of the voltage difference.
 3. Thecircuit of claim 2, wherein the pull-up transistors include: a firstpull-up transistor having a drain terminal coupled to the firstintermediate node and a gate terminal coupled to the second intermediatenode; and a second pull-up transistor having a drain terminal coupled tothe second intermediate node and a gate terminal coupled to the firstintermediate node; and wherein the pull-down transistors include: afirst pull-down transistor having a drain terminal coupled to the firstintermediate node and a gate terminal coupled to the second intermediatenode; and a second pull-down transistor having a drain terminal coupledto the second intermediate node and a gate terminal coupled to the firstintermediate node.
 4. The circuit of claim 1, wherein the senseamplifier further includes a cutoff transistor coupled between the firstand second paths to turn off responsive to activation of the latchcircuit.
 5. The circuit of claim 4, wherein the sense amplifier furtherincludes: a first feedback transistor with a drain terminal coupled to adrain terminal of the cutoff transistor and a gate terminal coupled tothe first intermediate node; and a second feedback transistor with adrain terminal coupled to a source terminal of the cutoff transistor anda gate terminal coupled to the second intermediate node.
 6. The circuitof claim 4, wherein the sense amplifier further includes: first outputlogic coupled to the first intermediate node to selectively pass a firstdigital value based on the voltage at the first intermediate noderesponsive to the enable signal; second output logic coupled to thesecond intermediate node to selectively pass a second digital valuebased on the voltage at the second intermediate node responsive to theenable signal, wherein the second digital value is the inverse of thefirst digital value; and cutoff logic to receive the first and seconddigital values as inputs and having an output terminal coupled to a gateterminal cutoff transistor to turn off the cutoff transistor responsiveto receipt of the first and second digital values.
 7. A memory systemcomprising: a resistive memory including a resistive memory cell; areference cell having a first resistance; a sense amplifier coupled tothe resistive memory cell and the reference cell, the sense amplifier toread data from the resistive memory cell based on the reference cell; afirst trimmable resistor coupled between the sense amplifier and theresistive memory cell; a second trimmable resistor coupled between thesense amplifier and the reference cell; and a control circuit to adjustresistance values of the first trimmable resistor and the secondtrimmable resistor to equalize read margins between a logic 0 read and alogic 1 read of the memory cell.
 8. The system of claim 7, wherein thereference cell is a first reference cell, wherein the system furthercomprises: a second reference cell with a second resistance that isdifferent from the first resistance; a third trimmable resistor coupledbetween the sense amplifier and the second reference cell; wherein thesense amplifier is to read the data from the resistive memory cell basedon the first and second reference cells; and wherein the control circuitis to adjust a resistance value of the third trimmable resistor toequalize the read margins.
 9. The system of claim 8, wherein the controlcircuit is to control the second and third trimmable resistors to havethe same resistance value.
 10. The system of claim 8, wherein the senseamplifier includes: a first path including a first intermediate node; asecond path including a second intermediate node; and switch circuitryto: during a first phase of the read, couple the first reference cell tothe first path and couple the resistive memory cell to the second path;and during a second phase of the read, couple the resistive memory cellto the first path and couple the second reference cell to the secondpath to generate a voltage difference between the first and secondintermediate nodes that corresponds to a value of a bit stored by theresistive memory cell.
 11. An apparatus comprising: means to, during afirst phase of a read operation, couple a reference cell to a first pathand couple a memory cell to a second path; means to, during a secondphase of the read operation, couple the memory cell with the first pathto generate a voltage difference between a first intermediate node onthe first path and a second intermediate node on the second path thatcorresponds to a value of a bit stored by the memory cell; and means togenerate a bias voltage to control a first transistor of the first pathto provide a voltage across the memory cell or reference cell and inducea current in the first path, the bias voltage having a value based on athreshold voltage of a second transistor.
 12. The apparatus of claim 11,wherein the means to generate the bias voltage is to copy a current onthe first path and pass the copied current through the second transistorto generate the bias voltage.
 13. The apparatus of claim 11, furthercomprising means to adjust an offset resistance associated with thememory cell or the reference cell to balance read margins of the readoperation.
 14. The apparatus of claim 11, further comprising means togenerate a digital output signal based on the generated voltagedifference between the first and second intermediate nodes, wherein themeans to generate the digital output signal are direct current (DC)coupled to the first and second intermediate nodes.